Photographing apparatus and photographing method

ABSTRACT

An image of a subject is focused on an image pickup device through a zoom lens and a diaphragm. The image focused on the image pickup device is photoelectrically converted into an electric signal. The electric signal as a picture signal is supplied to an amplifying circuit. The amplifying circuit amplifies the supplied picture signal. The amplified picture signal is digitized by an A/D converter. The digitized picture signal is supplied to a signal processing circuit. The signal processing circuit performs a clamping process, a color signal process, a luminance signal process, and other processes for such as a contour compensation, a defect compensation, and a white balance compensation. A camera CPU controls the zoom lens through a lens driving circuit, controls the image pickup device through a timing generating circuit, and controls an auxiliary lighting portion  7.  A signal received from an operating portion is supplied to the camera CPU.

TECHNICAL FIELD

[0001] The present invention relates to a photographing apparatus and a photographing method which allow a flashlight photographable range to be kept almost constant in lighting photography (such as a flashlight photography) even if a subject is photographed with a zoom lens of which a stop number of brightness (the amount of light which enters a photographing means) of the lens is varied corresponding to the focal distance of the lens.

BACKGROUND ART

[0002] Currently, most of cameras are provided with a flashlight as an auxiliary lighting means so that a subject can be photographed with a flashlight in a dirk place. Many of cameras having a built-in flashlight are provided with an automatic lighting function. Thus, with those cameras, everyone can easily photograph with a flashlight.

[0003] However, since a flashlight is built in a camera body, the amount of emitted light of the flashlight is limited. In addition, the maximum emitted light amount, which is so-called guide number (hereinafter abbreviated as GN) value, depends on the camera. The emitted light amount of the flashlight is proportion to the GN value.

[0004] Flashlight systems of most cameras have a light adjusting function. When the distance between a camera and a subject, namely, a focal distance, is in a flashlight photographable range, the light adjusting function automatically adjusts the emitted light amount of the flashlight so that the subject can be correctly exposed.

[0005] However, as the focal distance is apart from the flashlight photographable range, the subject is gradually under-exposed even with the maximum emitted light amount of the flashlight.

[0006] When a camera is provided with a zoom lens of which the brightness (F number) of the lens is varied corresponding to the focal distance (namely, the F number of the lens is dropped (hereinafter, referred to as F drop), the distance of which the subject is under-exposed at the focal distance (for example, wide end) of the brightest open F number is different from the distance of which the subject is under-exposed at the focal point (for example, tele end). As the F drop of the zoom lens is larger, the distance in which the subject is under-exposed becomes larger. When the subject is zoomed, the flashlight photographable range is largely varied.

[0007] To solve such a problem, Japanese Patent Laid-Open Publication No. SHO 56-17577 discloses an invention of which when a zoom magnification becomes large, the diaphragm is not open with the minimum stop number F which is necessary for obtaining a predetermined depth of field.

[0008] In addition, Japanese Patent Laid-Open Publication No. SHO 60-35717 discloses an invention of which a driving portion is disposed at a member which is moved by a zooming operation, a cam portion being disposed at an operating portion to which a diaphragm lever with which a stop-down operation is performed is rotatably disposed, the driving portion and the cum portion being engaged, when the driving portion is moved by the zooming operation, the operating member being driven and thereby the diaphragm lever being deformed, so the fluctuation of the stop number due to the zooming operation can be compensated.

[0009] In addition, a technology of which the distance to a subject is measured and the effective sensitivity of an image pickup device is increased corresponding to the distance is known. In addition, Japanese Patent Laid-Open Publication No. 2000-162679 describes an invention of which auxiliary lighting compensates the insufficiency of the emitted light amount of main lighting so as to increase the effective sensitivity of the image pickup device.

[0010] However, since the relation between the focal distance and the exposure is not considered in the flashlight photography, when the zooming operation is performed, an F drop takes place. As a result, the brightness of the lens is varied. The emitted light amount of the flashlight for correctly exposing the subject cannot be obtained depending on the focal distance. Thus, even if the subject is photographed with a zoom lens of which the brightness of the lens is varied depending on the focal distance, the flashlight photographable range is largely varied.

[0011] Therefore, an object of the present invention is to provide a photographing apparatus and a photographing method which allow a flashlight photographable range to be kept almost constant even if the brightness of a lens is varied due to an F drop when a zooming operation is performed.

DISCLOSURE OF THE INVENTION

[0012] Claim 1 of the present invention is a photographing apparatus, comprising auxiliary lighting means for lighting a subject; photographing means for photographing the subject; a zoom lens portion for varying the amount of light entered into the photographing means corresponding to a focal distance of the zoom lens portion; and amplifying means for amplifying a picture signal which is output from the photographing means with an amplification factor which has been set, wherein when the subject is photographed with the auxiliary lighting means, a setting of the amplification factor of the amplifying means is changed corresponding to the focal distance.

[0013] Claim 7 of the present invention is a photographing method, comprising the steps of causing lighting means to light a subject; causing photographing means to photograph the subject; amplifying a picture signal of the photographed subject with an amplification factor which has been set because an amount of light which enters the photographing means of a zoom lens portion is varied corresponding to a focal distance of the zoom lens portion; and changing a setting of the amplification factor corresponding to the focal distance.

[0014] In such a manner, the subject is lit by the lighting means. When the subject is photographed by the photographing means, since the amount of light (brightness of the lens) which enters the photographing means is varied depending on the focal distance, the picture signal of the photographed subject is amplified with the amplification factor which has been set. At that point, the setting of the amplification factor is changed depending on the focal distance. Thus, even if the brightness of the zoom lens portion is varied, the photographable range can be kept almost constant.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a block diagram showing an overall structure of a camera according to an embodiment of the present invention;

[0016]FIG. 2 is a characteristic diagram for explaining the relation between a stop number F of a lens and a distance D to a subject according to the present invention;

[0017]FIG. 3 is a characteristic diagram for explaining the present invention;

[0018]FIG. 4 is a flow chart for explaining a control according to an embodiment of the present invention;

[0019]FIG. 5 is a flow chart showing a first example of an amplitude factor setting control for compensating an F drop according to the present invention;

[0020]FIG. 6 is a flow chart showing a second example of an amplification factor setting control for compensating an F drop according to the present invention;

[0021]FIG. 7 is a flow chart showing a third example of an amplification factor setting control for compensating an F drop according to the present invention; and

[0022]FIG. 8 is a characteristic diagram showing an example of a variation of an amplification factor according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0023] Next, with reference to the accompanying drawings, an embodiment of the present invention will be described. For preventing redundant description, similar functional portions will be denoted by similar reference numerals in each of the accompanying drawings. FIG. 1 shows an overall structure of an embodiment of the present invention. An image of a subject is entered from the outside. The image is passed through a zoom lens 5 and a diaphragm 6 and then focused on an image pickup device 1. The image pickup device 1 is composed of for example a CCD (Charge Coupled Device). An image focused on the image pickup device 1 is photoelectrically converted into an electric signal. The electric signal is supplied as a picture signal to an amplifying circuit 2. The amplifying circuit 2 amplifies the supplied picture signal. As will be described later, an amplification factor of the amplifying circuit 2 is properly set. The picture signal amplified by the amplifying circuit 2 is supplied to an A/D converter 3. The A/D converter 3 digitizes the amplified picture signal. The digitized picture signal is supplied to a signal processing circuit 4. The signal processing circuit 4 performs a clamping process, a color signal process, a luminance signal process, and other processes for such as a contour compensation, a defect compensation, and a white balance compensation for the supplied picture signal.

[0024] The operations of those circuits are controlled by a camera CPU (Central Processing Unit) 11. For example, an automatic focus control of the zoom lens 5 is performed by the camera CPU 11 through a lens driving circuit 8. An automatic diaphragm control of the diaphragm 6 is performed by the camera CPU 11 through a diaphragm driving circuit 9. The image pickup device 1 is controlled by the camera CPU 11 through a timing generator 10. An auxiliary lighting portion 7 emits light for an amount (namely, adjusted light amount) corresponding to a control signal supplied from the camera CPU 11 at a predetermined timing. In addition, a signal which is output from an operating portion 12 operated by the user is supplied to the camera CPU 11. The operating portion 12 is used to set for example the amplification factor of the amplifying circuit 2 for the picture signal as will be described later.

[0025] When a subject is photographed with a flashlight using the auxiliary lighting portion 7, to correctly expose the subject, it is necessary to satisfy the following relation (formula 1):

GN=F·D  (formula 1)

[0026] where GN represents the GN value of the flashlight; F represents the stop number; and D represents the distance to the subject.

[0027] For example, assuming that the stop number is F2.8 and the distance D to the subject is 2 m, when the subject is flashlight photographed with an emitted light amount of which GN value is 5.6, the subject is correctly exposed.

[0028] The relation between the stop number F and the distance D to the subject is denoted by line 21 in FIG. 2. In this case, it is assumed that the GN value of the auxiliary lighting portion 7 is 10. Assuming that the line 21 is a boundary line, an area denoted by reference numeral 22 is a flashlight photographable range. An area denoted by reference numeral 23 is a flashlight non-photographable range.

[0029] In contrast, when the subject is flashlight photographed in the condition that the GN value of auxiliary lighting portion 7 is 10 and the stop number is F2.8, the distance D to the subject becomes around 3.6 m. The range of around 3.6 m is the flashlight photographable range. In other words, in the range of around 3.6 m, with the light adjusting function, the subject can be photographed with a correct exposure. However, when the distance D to the subject exceeds the flashlight photographable range (around 3.6 m), the subject is under-exposed.

[0030] When the GN value of the auxiliary lighting portion 7 is 10 and the stop number is F5.6, the distance D to the subject becomes around 1.8 m. The range of around 1.8 m is the flashlight photographable range. In the range of around 1.8 m, the subject can be flashlight photographed always with a correct exposure by the light adjusting function.

[0031] The GN value of the auxiliary lighting portion 7 disposed in the camera body cannot sufficiently correspond to the variation of the stop number F due to the limitation of the size and power consumption of the auxiliary lighting portion 7. Now, it is assumed that the camera is provided with the auxiliary lighting portion 7 whose GN value is 10 and the zoom lens 5 of which the brightness of the lens varies from F2.8 at the wide end to F5.6 at the tele end as an F drop. When a subject is flashlight photographed at the wide end by the camera with a distance of 3 m to the subject, it is correctly exposed. However, when the subject is zoomed, it is under exposed. At the tele end, unless the distance between the camera and the subject is decreased to around 1.8 m, the subject cannot be correctly exposed. In this case, although the subject has been zoomed in, it is necessary to cause the camera to further approach the subject.

[0032] From the view point of sensitivity, assuming that the image pickup device 1 outputs a picture signal based on for example ISO (International Organization for Standardization) speed 100, when a subject is zoomed, an F drop takes place. In this case, the sensitivity of a picture signal which is output from the image pickup device 1 is decreased for the decreased light. To compensate the decreased sensibility, the amplifying circuit 2 amplifies the picture signal with an amplification factor which has been set.

[0033] For example, as shown in FIG. 3A, when the stop number of the zoom lens 5 is F2.8, the image pickup device 1 outputs a picture signal of ISO speed 100. In this case, since the amplifying circuit 2 does not need to amplify the picture signal which is output from the image pickup device 1, as shown in FIG. 3B, the amplification factor is 0 dB.

[0034] In contrast, when the stop number of the zoom lens 5 is F4, to output a picture signal of ISO speed 100 at a stop number of F2.8 from the image pickup device 1, it is necessary to output a picture signal of ISO speed 200 from the image pickup device 1. Thus, the amplifying circuit 2 amplifies the picture signal which is output from the image pickup device 1 so that the picture signal becomes a signal of ISO speed 200. At that point, as shown in FIG. 3B, the amplification factor is 6 dB.

[0035] In addition, when the stop number of the zoom lens 5 is F5.6, to output a picture signal of ISO speed 100 at a stop number F2.8 from the image pickup device 1, it is necessary to output a picture signal of ISO speed 400 from the image pickup device 1. Thus, the amplifying circuit 2 amplifies the picture signal which is output from the image pickup device 1 so that the picture signal becomes a signal of ISO speed 400. At that point, as shown in FIG. 3B, the amplification factor is 12 dB.

[0036] The forgoing (formula 1) is based on ISO speed 100. When ISO speed S is considered, (formula 1) can be expressed with (formula 2).

[0037] ti GN·{square root}(S/100)=F·d  (Formula 2)

[0038] where {square root}( ) obtains the root of the calculated result in ( ).

[0039] Assuming that the ISO speed is 400, the GN value is 10, and the stop number is F5.6, the following relation can be obtained.

10·{square root}(400/100)=5.6·D

D≈3.57(m)

[0040] In other words, the flashlight photographable range becomes around 3.6 m.

[0041] Now, with reference to a flow chart shown in FIG. 4, an example of a control according to the embodiment will be described. At step S1, it is determined whether or not the shutter of the camera has been pressed halfway. When the determined result represents that the shutter has been pressed halfway, the flow advances to step S2. When the determined result represents that the shutter has not been pressed halfway, the flow cycles at step S1 until the shutter has been pressed halfway.

[0042] At step S2, it is determined whether or not flashlight photography is performed. When the determined result represents that flashlight photography is performed, the flow advances to step S3. When the determined result represents that flashlight photography is not performed, the flow advances to step S8. At that point, the brightness around the camera may be automatically measured. Corresponding to the measured result, it may be determined whether or not flashlight photography is set. Alternatively, flashlight photography may be set by user's operation.

[0043] At step S3, it is determined whether or not the shutter has been pressed deeply. When the determined result represents that the shutter has been pressed deeply, the flow advances to step S4. When the determined result represents that the shutter has not been pressed deeply, the flow returns to step S1.

[0044] At step S4, a subroutine which will be described later is called. In the subroutine, an amplification factor of the amplifying circuit 2 is set so that the decrease of the sensitivity of the image pickup device 1 due to an F drop is compensated. At step S5, the exposure of the image pickup device 1 is started. At step S6, the auxiliary lighting portion 7 emits light for a predetermined amount. At step S7, the exposure of the image pickup device 1 is finished. Thereafter, the control of the flow chart is finished.

[0045] When the determined result represents that flashlight photography is not performed, the flow advances to step S8. At step S8, it is determined whether or not the shutter has been pressed deeply. When the determined result represents that the shutter has been pressed deeply, the flow advances to step S9. When the determined result represents that the shutter has not been pressed deeply, the flow returns to step S1. At step S9, the exposure of the image pickup device 1 is started. Thereafter, the flow advances to step S7.

[0046] Next, with reference to a flow chart shown in FIG. 5, a first example of an amplification factor setting control for compensating an F drop at step S4 will be described. The flow chart shown in FIG. 5 represents a subroutine which is called when the flow advances to step S4.

[0047] At step S11, the GN value of the auxiliary lighting portion 7 and the stop number Fmin at which the zoom lens 5 becomes the brightest are obtained. At step S12, the maximum distance Dmax is calculated by the forgoing (formula 1). In other words, the distance D calculated by GN=Fmin·D becomes the maximum distance Dmax.

[0048] At step S13, the current stop number F of the zoom lens is detected. At step S14, the ISO speed S is calculated by the forgoing (formula 2). In other words, the ISO speed S is calculated by GN={square root}(S/100)=F·Dmax. At step S15, an amplification factor corresponding to the calculated ISO speed S is set.

[0049] After the subroutine of the flow chart shown in FIG. 5 is finished, the flow returns to step S4 at which the subroutine of the flow chart shown in FIG. 5 has been called.

[0050] Next, with reference to a flow chart shown in FIG. 6, a second example of the amplification factor setting control for compensating the F drop at step S4 will be described. The flow chart shown in FIG. 6 represents an example of which an amplification factor is set corresponding to the brightness. The flow chart shown in FIG. 6 represents a subroutine which is called when the flow advances to step S4.

[0051] At step S21, a reference value Eref with which brightness is compared is set. At step S22, the photometry means detects the brightness E around the camera. At step S23, it is determined whether or not the detected brightness E is higher than the reference value Eref. When the determined result represents that the detected brightness E is higher than the reference value Eref, the subroutine of the flow chart is finished. When the determined result represents that the detected brightness E is lower than the reference value Eref, the flow advances to step S24.

[0052] At step S24, the GN value of the auxiliary lighting portion 7 and the stop number Fmin at which the zoom lens 5 becomes the brightest are obtained. At step S25, with the obtained GN value and stop number Fmin, the maximum distance Dmax is calculated by the forgoing (formula 1). In other words, the maximum distance Dmax is calculated by GN=Fmin·Dmax.

[0053] At step S26, the current stop number F of the zoom lens 5 is detected. At step S27, the ISO speed S is calculated by the forgoing (formula 2). In other words, the ISO speed S is calculated by GN={square root}(S/100)=F·Dmax. At step S28, an amplification factor corresponding to the calculated ISO speed S is set.

[0054] When the subroutine of the flow chart shown in FIG. 6 is finished, the flow advances to step S4 at which the subroutine of the flow chart shown in FIG. 6 has been called. Thus, when the brightness E detected by the photometry means is higher than the reference value Eref, an amplification factor is not set.

[0055] Next, with reference to a flow chart shown in FIG. 7, a third example of the amplification factor setting control for compensating the F drop at step S4 will be described. The flow chart shown in FIG. 7 represents an example of which an amplification factor is set corresponding to the distance. The flow chart shown in FIG. 7 represents a subroutine which is called when the flow advances to step S4.

[0056] At step S31, the GN value of the auxiliary lighting portion 7 and the current stop number F of the zoom lens 5 are obtained. At step S32, with the obtained GN value and stop number F, the reference distance Dref is calculated by the forgoing (formula 1). In other words, the reference distance Dref is calculated by GN=F·Dref. At step S33, the distance d to the subject is detected by the distance measuring means.

[0057] At step S34, it is determined whether or not the subject is closer than the reference distance Dref. In other words, it is determined whether or not the distance D detected by the distance measuring means is equal to or smaller than the reference distance Dref. When the determined result represents that the detected distance D is equal to or smaller than the reference distance Dref, the subroutine of the flow chart is finished. When the determined result represents that the detected distance D is larger than the reference distance Dref, the flow advances to step S35.

[0058] At step S35, the stop number Fmin with which the zoom lens 5 becomes the brightest is obtained. At step S36, with the GN value obtained at step S31 and the stop number Fmin, the maximum distance Dmax is calculated by the forgoing (formula 1). In other words, the maximum distance Dmax is calculated by GN=Fmin·Dmax. At step S7, the ISO speed S is calculated by the forgoing (formula 2). In other words, the ISO speed S is calculated by GN={square root}(S/100)=F·Dmax. At step S38, an amplification factor corresponding to the calculated ISO speed S is set.

[0059] After the subroutine of the flow chart shown in FIG. 7 is finished, the flow returns to step S4 at which the subroutine of the flow chart shown in FIG. 7 has been called. Thus, when the distance D detected by the distance measuring means is equal to or smaller than the reference distance Dref, an amplification factor is not set.

[0060] When an amplification factor is set at step S4, one of the subroutines of the flow charts shown in FIGS. 5, 6, and 7 may be selected. When the brightness E detected by the photometry means is higher than the reference value Eref and when the distance D detected by the distance measuring means is equal to or smaller than the reference distance Dref, an amplification factor may not be set.

[0061] Now, the relation between the focal point and the amplification factor is shown in FIGS. 8A and 8B. As shown in FIG. 8A, the amplification factor may be increased in proportion to the focal distance. Alternatively, as shown in FIG. 8B, the amplification factor may be linearly increased corresponding to the focal distance. Alternatively, the amplification factor may be calculated by the forgoing (formula 2) whenever necessary. Alternatively, the amplification factor may be set stepwise or linearly in consideration of the balance with a real picture.

[0062] The relation between the focal distance and the amplification factor may be pre-set at factory before shipment. Alternatively, the relation may be freely set by the user. When the user sets the relation, he or she may set it in a menu format with the operating portion 12.

[0063] Now, it is assumed that the effective sensitivity of the image pickup device 1 is ISO speed 100 and the zoom lens 5 has a stop number in the range from F2.8 to F5.6. In this case, the amplification factor of the amplifying circuit 2 is set so that when the stop number becomes F5.6 as the result of the zooming operation, the effective sensitivity of the image pickup device 1 becomes equivalent to ISO speed 400. Consequently, a photographing distance at which the stop number is F2.8 is in the flashlight photographable range without need to approach the subject. In other words, the amount of light decreased due to the F drop can be compensated by varying the amplification factor.

[0064] However, if the amplification factor is increased corresponding to the amount of light which is decreased in proportion to the focal distance, since the noise component is also amplified, the quality of the supplied picture is deteriorated. Thus, it is important to balance the picture quality and correct exposure, not simply set the amplification factor corresponding to the focal distance. The upper limit of the amplification factor to be set may be up to for example ISO speed 200.

[0065] When a subject is flashlight photographed in a dirk place, no problem takes place. In contrast, when a subject is flashlight photographed in a bright room or at a sun shining place (namely, daylight synchronized flash photography is performed), if the amplification factor is thoughtlessly increased, stationary light is also amplified. As a result, the subject may be over-exposed. In this case, corresponding to information of which the brightness of the subject has been measured, a proper amplification factor may be set by determining whether to increase the amplification factor.

[0066] Even if an F drop takes place, when the distance is close to the subject and the light amount is sufficient, it is not necessary to increase the amplification factor. Thus, corresponding to information of which the distance to the subject has been measured, the amplification factor may be properly set.

[0067] According to the embodiment, the auxiliary lighting portion 7 is disposed in the camera. Alternatively, the auxiliary lighting portion 7 may be separated from the camera body.

[0068] According to the embodiment, the zoom lens 5 is disposed in the camera. Alternatively, the zoom lens 5 may be detachable from the camera body.

[0069] According to the present invention, when a subject is flashlight photographed with auxiliary lighting, an F drop of the zoom lens causes the amount of light which enters the image pickup device to be decreased. However, by varying the amplification factor, the decrease of the light amount can be compensated. Thus, even if the brightness of the lens is varied by the zooming operation, the flashlight photographable range can be kept almost constant.

[0070] According to the present invention, when a subject which is bright or which is at a close-up position is flashlight photographed with auxiliary lighting, an amplification factor with which a picture signal supplied from the image pickup device is amplified can be properly set regardless of the F drop of the zoom lens. Thus, over-exposure and occurrence of noise can be suppressed. 

1. A photographing apparatus, comprising: auxiliary lighting means for lighting a subject; photographing means for photographing the subject; a zoom lens portion for varying the amount of light entered into the photographing means corresponding to a focal distance of the zoom lens portion; and amplifying means for amplifying a picture signal which is output from the photographing means with an amplification factor which has been set, wherein when the subject is photographed with the auxiliary lighting means, a setting of the amplification factor of the amplifying means is changed corresponding to the focal distance.
 2. The photographing apparatus as set forth in claim 1, wherein the zoom lens portion is replaceable.
 3. The photographing apparatus as set forth in claim 1, further comprising: photometry means for measuring brightness of the subject, wherein when the brightness measured by the photometry means is higher than a predetermined value, a setting of the amplification factor is not changed.
 4. The photographing apparatus as set forth in claim 1, further comprising: distance measuring means for measuring a distance to the subject, wherein when the distance measured by the distance measuring means is equal to or smaller than a predetermined value, a setting of the amplification factor is not changed.
 5. The photographing apparatus as set forth in claim 1, further comprising: photometry means for measuring brightness of the subject; and distance measuring means for measuring a distance to the subject, wherein when the brightness measured by the photometry means is higher than a predetermined value and when the distance measured by the distance measuring means is equal to or smaller than a predetermined value, a setting of the amplification factor is not performed.
 6. The photographing apparatus as set forth in claim 1, wherein an upper limit of the amplification factor is set.
 7. A photographing method, comprising the steps of: causing lighting means to light a subject; causing photographing means to photograph the subject; amplifying a picture signal of the photographed subject with an amplification factor which has been set because an amount of light which enters the photographing means of a zoom lens portion is varied corresponding to a focal distance of the zoom lens portion; and changing a setting of the amplification factor corresponding to the focal distance.
 8. The photographing method as set forth in claim 7, further comprising the steps of: measuring brightness of the subject, and when the measured brightness is higher than a predetermined value, causing a setting of the amplification factor not to be changed.
 9. The photographing method as set forth in claim 7, further comprising the steps of: measuring a distance to the subject, and when the measured distance is equal to or smaller than a predetermined value, causing a setting of the amplification factor not to be changed.
 10. The photographing method as set forth in claim 7, further comprising the steps of: measuring brightness of the subject; measuring a distance to the subject; and when the measured brightness is higher than a predetermined value and when the measured distance is equal to or smaller than a predetermined value, causing a setting of the amplification factor not to be changed.
 11. The photographing method as set forth in claim 7, wherein an upper limit of the amplification factor is set. 